The present invention pertains to a steering configuration for providing omnidirectional maneuverability, and, more particularly, to a steering arrangement having at least two pairs of offset wheels.
A platform with omnidirectional mobility can move instantaneously in any specified direction across a surface from any current configuration of the platform. The word xe2x80x9carbitrary,xe2x80x9d as used in this application and in any appended claims, will mean xe2x80x9cas specified by a user.xe2x80x9d Thus, an omnidirectional vehicle can be said to be movable in an arbitrary direction on a continuously smooth surface.
Omnidirectional platforms or mobility bases provide obvious advantages in applications where a vehicle transporting a human subject or other load is to be used in congested rooms with static and/or dynamic obstacles and narrow aisles such as commonly found in nuclear plants, offices, factory workshops and warehouses, eldercare facilities and hospitals. Such platforms provide for enhanced maneuverability for mobile robots or automated vehicles in industry, military, personal, healthcare and other applications.
Various deficiencies are apparent in existing mobility designs. Classical wheeled mobile platform design, such as employed in three-wheel skid steering type mobile robots or in four-wheel car type mobile robots, suffers from limited mobility due to the non-holonomic constraints of the wheels. Hence, the motion of these vehicles is not truly omnidirectional. While such vehicles can reach any position and orientation in a plane, they need very complex maneuvers and require complicated path planning and control strategies in these environments. It is thus highly desirable for robots and vehicles to have omni-directional mobility for such applications.
Two approaches for achieving omnidirectional or near omnidirectional motion capability can be distinguished: special wheel design and conventional wheel design. Most special wheel designs are based on the universal wheel concept, which achieves traction in one direction and allows passive motion in another direction.
One type of special wheel design, called a ball wheel mechanism, is described by West and Asada, Design of Ball wheel Mechanisms for Omnidirectional Vehicles With Full Mobility and Invariant Kinematics, Journal of Mechanical Design, vol. 119, pp. 153-161 (June, 1997). In the design of West and Asada, two rings of rollers hold a solid ball. The power from a motor is transmitted, through gears meshed with teeth on an active ring, to the solid ball via friction between the roller and the ball. The other ring roller is mounted to the chassis and its rollers are free to rotate. Thus the ball will have a free motion around the ring axis as a result of the motion of other balls for the mobility platform. With a minimum of three such ball wheel assemblies an omnidirectional mobility platform can be constructed.
Universal wheel designs may exhibit good omnidirectional mobility however they tend to be complicated in terms of mechanical structure. Another major drawback of these designs is the limited load capacity for platforms built based on these designs because of the fact that the loads are supported by the slender rollers in the universal design or by the contact point with the floor in the orthogonal wheel and ball wheel designs. There are also sensitive to floor conditions as the surmountable height is limited by the small diameter of the rollers. The universal wheel design is also susceptible to vibrations as the rollers make successive contact with the ground. Additionally, these designs are not well-suited to carpeted or dirty floors because of the nature of their mechanisms.
Conventional wheels are inherently simple. As used herein, a xe2x80x9cconventional wheelxe2x80x9d refers to a rigid circular ring capable of rotation about a central transverse axis of rotation by virtue of mechanical coupling (as by spokes, for example) of the ring to an axle coaxial with the axis of rotation. Conventional wheels may have high load capacity and high tolerance to floor non-idealities such as bumps and cracks, dirt and debris. Various designs have been conceived to increase the mobility for platforms using conventional wheels. The most common designs are those using steered wheels. The platform has at least two active wheels with both driving and steering actuators. It can move at arbitrary directions from arbitrary configurations. But these type of systems are not truly omnidirectional because they need to stop and reorient the wheels to the desired direction whenever they need to travel in a trajectory with non-continuous curvatures.
One technique to use the conventional wheel for omnidirectional mobility is to use the active castor design as described by Wada and Mori, Holonomic and Omnidirectional Vehicle with Conventional Tires, Proc. IEEE Conf. on Robotics and Automation, pp. 3671-3676, (April, 1996). Wada and Mori describe an active wheel 10 fixed to a steering link 12, as shown in FIG. 1. Steering link 12 may be driven by a steering motor 14 and can rotate freely about a steering axis 16 fixed with respect to chassis 18 of the platform. Steering link 12 has an offset from the axis 20 of wheel 10, as shown. With at least two sets of such wheels omnidirectional mobility can be achieved for a platform. Active control of dual-wheel castors is described by Wada in U.S. Pat. No. 5,924,512 which is incorporated herein by reference.
In accordance with one aspect of the invention, in one of its embodiments, there is provided a mobile base for providing omnidirectional maneuverability. The mobile base has a rigid platform and at least two offset wheel assemblies. Each offset wheel assembly has two wheels having axles aligned along a common axis. The axles are free to rotate about an axis parallel to the planes of rotation of the wheels. Each offset assembly further includes a mechanical link that is pivotally coupled to a pivot point on the rigid platform, and the mechanical link supports the two wheels in such a manner that the common axis of the wheels is displaced from the pivot point. The assembly also includes a rotary actuator for independently driving each wheel.
In accordance with alternate embodiments of the invention, the common axis of the two wheels of each offset wheel assembly may be substantially perpendicular to a line connecting the midpoint between the centers of the wheels to the pivot point. The mobile base in accordance may also have a user input device for steering the mobile base in a specified direction and with a specified velocity. Additionally, the mobile base may have at least one sensor for sensing a velocity of a wheel. At least one passive wheel may be provided for supporting the platform.
In accordance with a further aspect of the present invention, there is provided an omnidirectional vehicle. The omnidirectional vehicle has a support for supporting a load and at least two offset wheel assemblies. Each offset wheel assembly has two wheels having axles aligned along a common axis. The axles are free to rotate about an axis parallel to the planes of rotation of the wheels. A mechanical link pivotally coupled to a pivot point on the support supports the two wheels in such a manrner that the common axis is displaced from the pivot point. A rotary actuator is included in each assembly for independently driving each wheel.
In accordance with another aspect of the present invention, an offset wheel assembly for providing omnidirectional maneuverability includes two wheels having axles aligned along a common axis. The axles are free to rotate about an axis parallel to the planes of rotation of the wheels. A mechanical link having a long axis supports the two wheels, and a rotary actuator drives each wheel independently.
In accordance with a further aspect of the present invention, a method for providing omnidirectional control of a vehicle having a platform includes providing at least two offset wheel assemblies. Each of the offset wheel assemblies has two wheels that share a common axis and a mechanical link pivotally coupled to an offset link joint on the platform. The mechanical link supports the two wheels in such a manner that the common axis is displaced from the offset link joint. A user specified platform velocity is received, and the user specified platform velocity vector is transformed to obtain a unique joint velocity for each of the offset link joints. Each offset link joint velocity vector is transformed to obtain a unique rotational velocity for each of the wheels, and a torque is applied to each wheel to cause each wheel to attain the unique rotational velocity. The user specified platform velocity vector is thus achieved. In addition, at least one rotary actuator may be driven when applying the torque.